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Ferromagnetism appears in nitrogen implanted nanocrystalline diamond films
Journal of Magnetism and Magnetic Materials 394 (2015) 477–480
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Ferromagnetism appears in nitrogen implanted nanocrystalline diamond films Zdenek Remes a, Shih-Jye Sun b,n, Marian Varga b, Hsiung Chou c, Hua-Shu Hsu d, Alexander Kromka b, Pavel Horak e a
Institute of Physics ASCR v.v.i., Cukrovarnicka 10, 162 00 Prague 6, Czech Republic Department of Applied Physics, National University of Kaohsiung, Kaohsiung 811, Taiwan c Department of Physics, National Sun Yat-Sen University, Kaohsiung 804, Taiwan d Department of Applied Physics, National Pingtung University of Education, Pingtung 900, Taiwan e Nuclear Physics Institute, 250 68 Rez, Czech Republic b
art ic l e i nf o
a b s t r a c t
Article history: Received 6 March 2015 Received in revised form 2 July 2015 Accepted 12 July 2015 Available online 14 July 2015
The nanocrystalline diamond films turn to be ferromagnetic after implanting various nitrogen doses on them. Through this research, we confirm that the room-temperature ferromagnetism of the implanted samples is derived from the measurements of magnetic circular dichroism (MCD) and superconducting quantum interference device (SQUID). Samples with larger crystalline grains as well as higher implanted doses present more robust ferromagnetic signals at room temperature. Raman spectra indicate that the small grain-sized samples are much more disordered than the large grain-sized ones. We propose that a slightly large saturated ferromagnetism could be observed at low temperature, because the increased localization effects have a significant impact on more disordered structure. & 2015 Elsevier B.V. All rights reserved.
Keywords: Nanocrystalline diamond films Nitrogen implanted diamond films
1. Introduction Diamond, a carbon (C) based material with high band gap semiconductor [1,2], possesses many excellent characteristics for the electronic and industrial applications, such as extremely high thermal conductivity, high carrier mobility, high reflection index and high mechanical hardness. A progress in the microwave plasma enhanced chemical vapor deposition (MW PECVD) [3] was allowed to prepare polycrystalline and nanocrystalline diamond (NCD) films [4–6] with an excellent optical quality for possible optoelectronic applications [7–9]. The MW PECVD diamond films are purer than the natural diamond ones, because the extrinsically impure elements will be excluded from the composition except some inevitable hydrogencarbon contaminations. To be different in the polycrystalline diamond films, the grain size in the NCD films is below 100 μm [10–12]. It is expected that plenty of boundaries existing in the films influence the electronic and optical properties as well [13–15]. The C element as same as silicon and germanium elements belongs to group IV in the periodic table, which can be used to be the semiconductor electronics. Unfortunately, owing to a very large band gap, the electronic properties of the diamond are rarely n
Corresponding author. Fax: þ 886 7 5919357. E-mail address: [email protected] (S.-J. Sun).
http://dx.doi.org/10.1016/j.jmmm.2015.07.034 0304-8853/& 2015 Elsevier B.V. All rights reserved.
modified by implanting different valence elements [16,17]. However, the large band gap makes the diamond to have the potential of becoming the material of high Curie temperature spintronics [18–20]. In general, the ferromagnetism of semiconductors arises from the doped magnetic ions. Specifically, the ferromagnetism can be manipulated in the diamond films without any magnetic ions doping, for instance, the nitrogen (N) doped diamond films [21–23]. Doping N to diamond is n-type doping due to the valence charge difference between C and N elements. It is confirmed that the doping states are deep from the first principles calculations [24]. In fact, it is very hard to vary the conductivity from such deep doping states. The samples of MW PECVD diamond possess two different sizes of nanoscaled grain. Here, we introduced N to those samples by means of implanting N ions to the host films. These samples exhibit the implant dose as well as the grain size with dependent ferromagnetism at room (high) and low temperatures. Actually, some carbon based compounds have been investigated [25], such as graphene which exhibits ferromagnetism from defects and structure edges. Since the insulating diamond film is very different from the conductive graphene, its ferromagnetic mechanism is still unknown. In this study, based on various experimental results, we demonstrate the magnetic properties from our NCD films and further propose a model to interpret the ferromagnetic mechanism in the nonmagnetic ions doped diamond films.
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Z. Remes et al. / Journal of Magnetism and Magnetic Materials 394 (2015) 477–480
2. Sample preparation and measurements Diamond-thin films were grown on 10 10 mm2 large Corning Eagle2000 glass. Glass substrates were seeded by applying ultrasonic agitation in a water-based diamond powder suspension (powder from NanoAmando, New Metals and Chemicals Corp. Ltd., Kyobashi). The typical density of seeds after such a process is in a range up to 1011 cm 2 [26]. Diamond films were grown through the plasma CVD process with largely pulsed linear-antenna microwave (modified system AK 400, Roth and Rau, AG), which is derived from the CH4/CO2/H2 gas mixture [27]. The conditions of diamond growth were 2.5% of CH4 and 10% CO2 compared with hydrogen, 2 pulsed microwave power of 2 kW and a substrate temperature of 650 °C. Two different sets of diamond films, separated into smaller and larger grain sizes, were achieved by controlling the total pressure of gas mixture during the diamond growth. The small grain-sized diamond film was deposited at the gas pressure of 100 Pa for 18 h, meanwhile the larger grain-sized diamond film at 8 Pa for 16 h. Decrease of the gas pressure results in more effective plasma spreading in the vacuum chamber, which further results in the highest optical quality (less sp2 phases) for diamond films grown [28]. The thicknesses of deposited diamond films were evaluated from the interference fringes of the reflectance spectra measured in UV–vis–NIR region and a commercial software for modeling the optical properties of thin films (FilmWizard) [29]. The thicknesses of the smaller and larger grainsized diamond films were evaluated to 260 nm and 320 nm, respectively. The NCD films were irradiated in NPI ASCR Rez [X] with lowenergy nitrogen ions using the electrostatic accelerator to be equipped with the Duoplasmatron ion source. The ion facility was used without a separated magnet, i.e. the NCD films were implanted mainly by the mixture of N þ and N2 þ ions. For implantation, 99.99 % pure nitrogen gas was applied into the Duoplasmatron. The nitrogen ions were accelerated to the nominal energy of 35 keV, so the range of the N þ ions in the NCD film achieved ∼50 nm. During implantation, the ion beam current was kept at low (below 100 nA), the flux of the nitrogen ions was selected from 1011 to 10 16 per cm2. The NCD samples (of the size 10 10 mm2 ) were irradiated in four separated circular spots (selected by an aperture), each of the 3 mm area in diameter, with different flux. The ferromagnetism dependent on temperature was evaluated from the superconducting quantum interference device (SQUID) measurements. The magnetic circular dichroism (MCD) with the Jasco J-815 spectrometer is equipped with 450 W Xenon lamp. The white light was incident upon a monochromator, which output linearly polarized light. This light then passed through a photoelastic modulator, which acted as a quarter wave plate and changed the linearly polarized light to the circularly polarized one. The light also switched frequency to 50 kHz so as to produce both right and left circularly polarized light and further allow for simultaneous recording of the MCD and optical density spectra.
Table 1 Four samples with different grain sizes and implant doses. Grain size/Implant dose
N-dose 7 1011/cm2
N-dose 7 1014/cm2
Grain size > 100 nm Grain size < 10 nm
Sample No. 4 Sample No. 9
Sample No. 5 Sample No. 10
Fig. 1. The MCD signals of all samples at room temperature.
energy 5.5 eV, which is consistent with the band edge energy of the diamond, as shown in Fig. 1, and the measurements reveal the fact that the ferromagnetism arises from the diamond matrix itself, instead of some non-correlated magnetic sources. Actually, it is difficult for the magnitude of MCD signal to precisely verify the magnitude of the magnetization. Based on a quantitative analysis, we employed the SQUID to identify the ferromagnetism. The ferromagnetism at room temperature was measured from SQUID, as shown in Fig. 2, which confirms that the ferromagnetic signals measured by MCD are real. Furthermore, at very low temperature, i.e., 5 K, all samples almost show a common saturated magnetization, as shown in Fig. 3. If we take a close look at these measured curves, we could find that the small grain-sized samples show slightly larger saturated magnetization. However, even at very low temperature, the hysteresis loops in all samples are small, which indicates a very weak correlation between the magnetic domains. Obviously, at the room (high) temperatures the implanted dose and the grain size are two factors to dominate the magnetization of samples. Fig. 2 shows that the magnitude of the magnetization of the same grain-sized samples increases with the implanted dose at room temperatures, which is particular for the large grain-sized samples, i.e., Nos. 4 and 5. This trend is indistinct for very small grain-sized samples, which implies that the rich grain boundaries will suppress the ferromagnetism. In view of this condition, we suppose that many Ns are located on the
3. Results and discussion The four samples in this study with different grain sizes as well as implanted doses were indexed by Nos. 4, 5, 9 and 10, as shown in the Table 1. Sample Nos. 4 and 5 possess the large grain size (>100 nm ) with different implant doses, 7 1011 and 7 1014, respectively. Contrast to the large grain-sized samples above, the average grain size of Nos. 9 and 10 is less than 10 nm. The ferromagnetic signals from all samples measured by the optical MCD at room temperature were found residing on the
Fig. 2. The room-temperature magnetic moment (emu/cm2) as a function of magnetic field for all the four nitrogen irradiated samples.
Z. Remes et al. / Journal of Magnetism and Magnetic Materials 394 (2015) 477–480
Fig. 3. The low-temperature magnetic moment (emu/cm2) as a function of magnetic field for all the four nitrogen irradiated samples.
boundaries, rather than being inside the diamond matrix. Besides, the common saturated magnetization at very low temperature implies that the dominant mechanism is different between high and low temperatures. In order to further investigate the implanted effect on the ferromagnetism, we annealed the large and small grain-sized samples, No. 5 and No. 9, respectively, in the vacuum and at 150 °C low temperature. Figs. 4 and 5 represent the MCD signals for samples before and after annealing, respectively. We summed up the left and right polarized signals to obtain the total density of states. In the process, we found that the low-temperature annealing made both samples to appear a new signal at ∼5.2 eV, marked by the arrow, which indicates that the annealing process will induce a deep impurity state relative to the band edge energy 5.5 eV. It is worth noting that the original ferromagnetic signals at the band edge of both different grain sized samples will be significant owing to the appearance of a new impurity state. This significance also implies that the deep impurity state is closely related to the implant N as well as the ferromagnetism induction. The appearance of the new impurity state also indicates that the original implant N is interstitially embedded in the films before the annealing. Thus, the ferromagnetism induced from the implant N dose does not completely arise from one doping, neither interstitial nor substituent. Implant dose and valence charge of dopant, which both make the structure disorder, seem to be the two key factors to induce the ferromagnetism from diamond films. Due to the decrease of the long range order, NCD is different from the microcrystalline diamond and possesses a lot of fractals to result in many sp3 bonds breaking to sp2 bonds. Thus, the NCD is the mixture of dominant sp3 and partial sp2 bonds [31]. Raman spectrum measurement is a good method to specifically evaluate the various bonds in a material. In addition to a small diamond peak at 1332 cm 1, the spectrum in a typical NCD has four extra
Fig. 4. The MCD signals from the large grain-sized sample before and after 150 °C vacuum annealing. The marked arrow indicates a new impurity state at ∼5.2 eV.
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Fig. 5. The MCD signals from the small grain-sized sample before and after 150 °C vacuum annealing. The marked arrow indicates a new impurity state at ∼5.2 eV.
features at 1150, 1350, 1480 and 1550 cm 1 [32,33]. The peaks at 1350 cm 1 and 1550 cm 1 are the D and G signals, respectively, which generally appear in the amorphous carbon with graphite components. In particular, the D band is strongly related to the degree of freedom of disorder in materials. In fact, the Raman spectra in the N doped NCD samples should be deviated from typical NCD because of the vibrational perturbation from N as well as the structure damage caused by the implant impacts. D band signals from small grain-sized samples, Nos. 9 and 10, are much more significant than those from large grain-sized ones, Nos. 4 and 5, as shown in Fig. 6, which both imply that the disorder is much more serious in small grain-sized NCD samples. Actually, our small grain-sized NCD samples show the similar Raman spectrum with other groups, of which the signals can be clearly verified from the essential diamond, the disordered graphite structure and hydrogenated carbon compounds, i.e. transpolyacetylene [30]. Instead, the Raman spectrum in our large grain-sized NCD samples deviates typical NCD with some undefined signal peaks. We suppose that the signal deviation results from some structure vacancies created by N implants. Furthermore, the sp3 bonds in large grainsized NCD samples are much richer than those in small grain-sized samples, because their Raman spectra show very weak D and G signals which will be enhanced by the irradiation from visible Raman. In short, the large grain-sized NCD samples have the microstructure variation by the participation of N and, through the variation, cause the ferromagnetism to appear. Herein, we propose a model to interpret how the ferromagnetism appears in the N doped NCD films. The implanted N elements, interstitially embedded on the carbon matrix of the diamond, are very close to C elements. The electron clouds of N and C elements extrude each other and jointly result in the
Fig. 6. The visible Raman spectrum for all samples. The peaks ν1 and ν3 indicate 1150 cm 1 and 1480 cm 1, respectively. The peaks at 1350 cm 1 and 1550 cm 1 are corresponding to the D and G modes, respectively.
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increase of the Coulomb repulsions within these close elements. However, the Coulomb effect on C elements is relatively less than that on N, because C elements possess an empty p orbit which provides excessive degree of freedom for the electron clouds. Furthermore, the empty p orbits on C elements also facilitate the p electron clouds of N elements to the extent of empty spaces. This extension causes an induced dipole moment in N. Consequently, a crystal field, established along the direction of the empty orbit, splits the triple degenerate energy levels into two separated levels. Since there are three electrons on p orbits of N, two levels are double degenerate with half a local spin in the other level as a result. Therefore, we suppose that the ferromagnetism arises from the local spins which vastly distribute in NCD films and closely correlate with one another. Besides, the grain boundaries provide more degree of freedom to the structural disorder, particularly for the elements nearby the boundaries. In the boundaries many sp3 bonds break to sp2, which results in reducing the orbital Coulomb repulsion and the ferromagnetism. In the model, the magnitude of ferromagnetism increases with N implanted doses, as we observe the experimental results at room temperature. Besides, it is expected that implanted N to the NCD samples causes the structure distortion and the charge localization. This carrier localization is not proportionally related to the implanted dose, because high implanted dose does not mean that the distortion is rather serious, too. The structure distortion will increase the Coulomb repulsion to induce the ferromagnetism, which is dominant at low temperature, with difference at high temperature. Based on the Raman experiments, the small grain-sized NCD samples should be much more distorted, which is consistent with the result that the ferromagnetism of small grainsized samples is slightly larger than that of large grain-sized ones while being at low temperature.
4. Conclusion Our nanocrystalline diamond films with different grain sizes, prepared by pulsed linear antenna microwave plasma enhanced chemical vapor deposition method, exhibit the clear ferromagnetism at room temperature as nitrogen element was implanted to the samples. Besides, large grain-sized samples obviously show more robust ferromagnetism than small grain-sized samples. On the other hand, the magnitude of ferromagnetism increases with the implanted dose. For the large grain-sized samples, the nitrogen implants made a serious Raman deviation from the typical nanocrystalline diamond films. The ferromagnetism we proposed arises from a crystal field established by the dipole moments on nitrogen which splits the degenerate energy levels. From the Raman spectrum, the structure of small grain-sized samples is more disordered than that of large grainsized samples, and the spectrum of small grain-sized samples is similar to that of the typical nanocrystalline diamond films. Finally, we find that the ferromagnetism induced from the structure distortion is dynamically dominant at low temperature, which leads to the result that the ferromagnetism of small grain-sized samples is slightly larger than that of large grain-sized ones.
Acknowledgments We would like to thank the support by the Ministry of Science and Technology of Republic of China for the Grant no. NSC-1012112-M-390-001-MY3 (Shih-Jye Sun), the Project no. P108/12/ G108 (GACR), LD14011 (HINT COST Action MP1202). The ion implantation was performed at the CANAM infrastructure of the NPI ASCR Rez.
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Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Ferromagnetism appears in nitrogen implanted nanocrystalline diamond films Zdenek Remes a, Shih-Jye Sun b,n, Marian Varga b, Hsiung Chou c, Hua-Shu Hsu d, Alexander Kromka b, Pavel Horak e a
Institute of Physics ASCR v.v.i., Cukrovarnicka 10, 162 00 Prague 6, Czech Republic Department of Applied Physics, National University of Kaohsiung, Kaohsiung 811, Taiwan c Department of Physics, National Sun Yat-Sen University, Kaohsiung 804, Taiwan d Department of Applied Physics, National Pingtung University of Education, Pingtung 900, Taiwan e Nuclear Physics Institute, 250 68 Rez, Czech Republic b
art ic l e i nf o
a b s t r a c t
Article history: Received 6 March 2015 Received in revised form 2 July 2015 Accepted 12 July 2015 Available online 14 July 2015
The nanocrystalline diamond films turn to be ferromagnetic after implanting various nitrogen doses on them. Through this research, we confirm that the room-temperature ferromagnetism of the implanted samples is derived from the measurements of magnetic circular dichroism (MCD) and superconducting quantum interference device (SQUID). Samples with larger crystalline grains as well as higher implanted doses present more robust ferromagnetic signals at room temperature. Raman spectra indicate that the small grain-sized samples are much more disordered than the large grain-sized ones. We propose that a slightly large saturated ferromagnetism could be observed at low temperature, because the increased localization effects have a significant impact on more disordered structure. & 2015 Elsevier B.V. All rights reserved.
Keywords: Nanocrystalline diamond films Nitrogen implanted diamond films
1. Introduction Diamond, a carbon (C) based material with high band gap semiconductor [1,2], possesses many excellent characteristics for the electronic and industrial applications, such as extremely high thermal conductivity, high carrier mobility, high reflection index and high mechanical hardness. A progress in the microwave plasma enhanced chemical vapor deposition (MW PECVD) [3] was allowed to prepare polycrystalline and nanocrystalline diamond (NCD) films [4–6] with an excellent optical quality for possible optoelectronic applications [7–9]. The MW PECVD diamond films are purer than the natural diamond ones, because the extrinsically impure elements will be excluded from the composition except some inevitable hydrogencarbon contaminations. To be different in the polycrystalline diamond films, the grain size in the NCD films is below 100 μm [10–12]. It is expected that plenty of boundaries existing in the films influence the electronic and optical properties as well [13–15]. The C element as same as silicon and germanium elements belongs to group IV in the periodic table, which can be used to be the semiconductor electronics. Unfortunately, owing to a very large band gap, the electronic properties of the diamond are rarely n
Corresponding author. Fax: þ 886 7 5919357. E-mail address: [email protected] (S.-J. Sun).
http://dx.doi.org/10.1016/j.jmmm.2015.07.034 0304-8853/& 2015 Elsevier B.V. All rights reserved.
modified by implanting different valence elements [16,17]. However, the large band gap makes the diamond to have the potential of becoming the material of high Curie temperature spintronics [18–20]. In general, the ferromagnetism of semiconductors arises from the doped magnetic ions. Specifically, the ferromagnetism can be manipulated in the diamond films without any magnetic ions doping, for instance, the nitrogen (N) doped diamond films [21–23]. Doping N to diamond is n-type doping due to the valence charge difference between C and N elements. It is confirmed that the doping states are deep from the first principles calculations [24]. In fact, it is very hard to vary the conductivity from such deep doping states. The samples of MW PECVD diamond possess two different sizes of nanoscaled grain. Here, we introduced N to those samples by means of implanting N ions to the host films. These samples exhibit the implant dose as well as the grain size with dependent ferromagnetism at room (high) and low temperatures. Actually, some carbon based compounds have been investigated [25], such as graphene which exhibits ferromagnetism from defects and structure edges. Since the insulating diamond film is very different from the conductive graphene, its ferromagnetic mechanism is still unknown. In this study, based on various experimental results, we demonstrate the magnetic properties from our NCD films and further propose a model to interpret the ferromagnetic mechanism in the nonmagnetic ions doped diamond films.
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Z. Remes et al. / Journal of Magnetism and Magnetic Materials 394 (2015) 477–480
2. Sample preparation and measurements Diamond-thin films were grown on 10 10 mm2 large Corning Eagle2000 glass. Glass substrates were seeded by applying ultrasonic agitation in a water-based diamond powder suspension (powder from NanoAmando, New Metals and Chemicals Corp. Ltd., Kyobashi). The typical density of seeds after such a process is in a range up to 1011 cm 2 [26]. Diamond films were grown through the plasma CVD process with largely pulsed linear-antenna microwave (modified system AK 400, Roth and Rau, AG), which is derived from the CH4/CO2/H2 gas mixture [27]. The conditions of diamond growth were 2.5% of CH4 and 10% CO2 compared with hydrogen, 2 pulsed microwave power of 2 kW and a substrate temperature of 650 °C. Two different sets of diamond films, separated into smaller and larger grain sizes, were achieved by controlling the total pressure of gas mixture during the diamond growth. The small grain-sized diamond film was deposited at the gas pressure of 100 Pa for 18 h, meanwhile the larger grain-sized diamond film at 8 Pa for 16 h. Decrease of the gas pressure results in more effective plasma spreading in the vacuum chamber, which further results in the highest optical quality (less sp2 phases) for diamond films grown [28]. The thicknesses of deposited diamond films were evaluated from the interference fringes of the reflectance spectra measured in UV–vis–NIR region and a commercial software for modeling the optical properties of thin films (FilmWizard) [29]. The thicknesses of the smaller and larger grainsized diamond films were evaluated to 260 nm and 320 nm, respectively. The NCD films were irradiated in NPI ASCR Rez [X] with lowenergy nitrogen ions using the electrostatic accelerator to be equipped with the Duoplasmatron ion source. The ion facility was used without a separated magnet, i.e. the NCD films were implanted mainly by the mixture of N þ and N2 þ ions. For implantation, 99.99 % pure nitrogen gas was applied into the Duoplasmatron. The nitrogen ions were accelerated to the nominal energy of 35 keV, so the range of the N þ ions in the NCD film achieved ∼50 nm. During implantation, the ion beam current was kept at low (below 100 nA), the flux of the nitrogen ions was selected from 1011 to 10 16 per cm2. The NCD samples (of the size 10 10 mm2 ) were irradiated in four separated circular spots (selected by an aperture), each of the 3 mm area in diameter, with different flux. The ferromagnetism dependent on temperature was evaluated from the superconducting quantum interference device (SQUID) measurements. The magnetic circular dichroism (MCD) with the Jasco J-815 spectrometer is equipped with 450 W Xenon lamp. The white light was incident upon a monochromator, which output linearly polarized light. This light then passed through a photoelastic modulator, which acted as a quarter wave plate and changed the linearly polarized light to the circularly polarized one. The light also switched frequency to 50 kHz so as to produce both right and left circularly polarized light and further allow for simultaneous recording of the MCD and optical density spectra.
Table 1 Four samples with different grain sizes and implant doses. Grain size/Implant dose
N-dose 7 1011/cm2
N-dose 7 1014/cm2
Grain size > 100 nm Grain size < 10 nm
Sample No. 4 Sample No. 9
Sample No. 5 Sample No. 10
Fig. 1. The MCD signals of all samples at room temperature.
energy 5.5 eV, which is consistent with the band edge energy of the diamond, as shown in Fig. 1, and the measurements reveal the fact that the ferromagnetism arises from the diamond matrix itself, instead of some non-correlated magnetic sources. Actually, it is difficult for the magnitude of MCD signal to precisely verify the magnitude of the magnetization. Based on a quantitative analysis, we employed the SQUID to identify the ferromagnetism. The ferromagnetism at room temperature was measured from SQUID, as shown in Fig. 2, which confirms that the ferromagnetic signals measured by MCD are real. Furthermore, at very low temperature, i.e., 5 K, all samples almost show a common saturated magnetization, as shown in Fig. 3. If we take a close look at these measured curves, we could find that the small grain-sized samples show slightly larger saturated magnetization. However, even at very low temperature, the hysteresis loops in all samples are small, which indicates a very weak correlation between the magnetic domains. Obviously, at the room (high) temperatures the implanted dose and the grain size are two factors to dominate the magnetization of samples. Fig. 2 shows that the magnitude of the magnetization of the same grain-sized samples increases with the implanted dose at room temperatures, which is particular for the large grain-sized samples, i.e., Nos. 4 and 5. This trend is indistinct for very small grain-sized samples, which implies that the rich grain boundaries will suppress the ferromagnetism. In view of this condition, we suppose that many Ns are located on the
3. Results and discussion The four samples in this study with different grain sizes as well as implanted doses were indexed by Nos. 4, 5, 9 and 10, as shown in the Table 1. Sample Nos. 4 and 5 possess the large grain size (>100 nm ) with different implant doses, 7 1011 and 7 1014, respectively. Contrast to the large grain-sized samples above, the average grain size of Nos. 9 and 10 is less than 10 nm. The ferromagnetic signals from all samples measured by the optical MCD at room temperature were found residing on the
Fig. 2. The room-temperature magnetic moment (emu/cm2) as a function of magnetic field for all the four nitrogen irradiated samples.
Z. Remes et al. / Journal of Magnetism and Magnetic Materials 394 (2015) 477–480
Fig. 3. The low-temperature magnetic moment (emu/cm2) as a function of magnetic field for all the four nitrogen irradiated samples.
boundaries, rather than being inside the diamond matrix. Besides, the common saturated magnetization at very low temperature implies that the dominant mechanism is different between high and low temperatures. In order to further investigate the implanted effect on the ferromagnetism, we annealed the large and small grain-sized samples, No. 5 and No. 9, respectively, in the vacuum and at 150 °C low temperature. Figs. 4 and 5 represent the MCD signals for samples before and after annealing, respectively. We summed up the left and right polarized signals to obtain the total density of states. In the process, we found that the low-temperature annealing made both samples to appear a new signal at ∼5.2 eV, marked by the arrow, which indicates that the annealing process will induce a deep impurity state relative to the band edge energy 5.5 eV. It is worth noting that the original ferromagnetic signals at the band edge of both different grain sized samples will be significant owing to the appearance of a new impurity state. This significance also implies that the deep impurity state is closely related to the implant N as well as the ferromagnetism induction. The appearance of the new impurity state also indicates that the original implant N is interstitially embedded in the films before the annealing. Thus, the ferromagnetism induced from the implant N dose does not completely arise from one doping, neither interstitial nor substituent. Implant dose and valence charge of dopant, which both make the structure disorder, seem to be the two key factors to induce the ferromagnetism from diamond films. Due to the decrease of the long range order, NCD is different from the microcrystalline diamond and possesses a lot of fractals to result in many sp3 bonds breaking to sp2 bonds. Thus, the NCD is the mixture of dominant sp3 and partial sp2 bonds [31]. Raman spectrum measurement is a good method to specifically evaluate the various bonds in a material. In addition to a small diamond peak at 1332 cm 1, the spectrum in a typical NCD has four extra
Fig. 4. The MCD signals from the large grain-sized sample before and after 150 °C vacuum annealing. The marked arrow indicates a new impurity state at ∼5.2 eV.
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Fig. 5. The MCD signals from the small grain-sized sample before and after 150 °C vacuum annealing. The marked arrow indicates a new impurity state at ∼5.2 eV.
features at 1150, 1350, 1480 and 1550 cm 1 [32,33]. The peaks at 1350 cm 1 and 1550 cm 1 are the D and G signals, respectively, which generally appear in the amorphous carbon with graphite components. In particular, the D band is strongly related to the degree of freedom of disorder in materials. In fact, the Raman spectra in the N doped NCD samples should be deviated from typical NCD because of the vibrational perturbation from N as well as the structure damage caused by the implant impacts. D band signals from small grain-sized samples, Nos. 9 and 10, are much more significant than those from large grain-sized ones, Nos. 4 and 5, as shown in Fig. 6, which both imply that the disorder is much more serious in small grain-sized NCD samples. Actually, our small grain-sized NCD samples show the similar Raman spectrum with other groups, of which the signals can be clearly verified from the essential diamond, the disordered graphite structure and hydrogenated carbon compounds, i.e. transpolyacetylene [30]. Instead, the Raman spectrum in our large grain-sized NCD samples deviates typical NCD with some undefined signal peaks. We suppose that the signal deviation results from some structure vacancies created by N implants. Furthermore, the sp3 bonds in large grainsized NCD samples are much richer than those in small grain-sized samples, because their Raman spectra show very weak D and G signals which will be enhanced by the irradiation from visible Raman. In short, the large grain-sized NCD samples have the microstructure variation by the participation of N and, through the variation, cause the ferromagnetism to appear. Herein, we propose a model to interpret how the ferromagnetism appears in the N doped NCD films. The implanted N elements, interstitially embedded on the carbon matrix of the diamond, are very close to C elements. The electron clouds of N and C elements extrude each other and jointly result in the
Fig. 6. The visible Raman spectrum for all samples. The peaks ν1 and ν3 indicate 1150 cm 1 and 1480 cm 1, respectively. The peaks at 1350 cm 1 and 1550 cm 1 are corresponding to the D and G modes, respectively.
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increase of the Coulomb repulsions within these close elements. However, the Coulomb effect on C elements is relatively less than that on N, because C elements possess an empty p orbit which provides excessive degree of freedom for the electron clouds. Furthermore, the empty p orbits on C elements also facilitate the p electron clouds of N elements to the extent of empty spaces. This extension causes an induced dipole moment in N. Consequently, a crystal field, established along the direction of the empty orbit, splits the triple degenerate energy levels into two separated levels. Since there are three electrons on p orbits of N, two levels are double degenerate with half a local spin in the other level as a result. Therefore, we suppose that the ferromagnetism arises from the local spins which vastly distribute in NCD films and closely correlate with one another. Besides, the grain boundaries provide more degree of freedom to the structural disorder, particularly for the elements nearby the boundaries. In the boundaries many sp3 bonds break to sp2, which results in reducing the orbital Coulomb repulsion and the ferromagnetism. In the model, the magnitude of ferromagnetism increases with N implanted doses, as we observe the experimental results at room temperature. Besides, it is expected that implanted N to the NCD samples causes the structure distortion and the charge localization. This carrier localization is not proportionally related to the implanted dose, because high implanted dose does not mean that the distortion is rather serious, too. The structure distortion will increase the Coulomb repulsion to induce the ferromagnetism, which is dominant at low temperature, with difference at high temperature. Based on the Raman experiments, the small grain-sized NCD samples should be much more distorted, which is consistent with the result that the ferromagnetism of small grainsized samples is slightly larger than that of large grain-sized ones while being at low temperature.
4. Conclusion Our nanocrystalline diamond films with different grain sizes, prepared by pulsed linear antenna microwave plasma enhanced chemical vapor deposition method, exhibit the clear ferromagnetism at room temperature as nitrogen element was implanted to the samples. Besides, large grain-sized samples obviously show more robust ferromagnetism than small grain-sized samples. On the other hand, the magnitude of ferromagnetism increases with the implanted dose. For the large grain-sized samples, the nitrogen implants made a serious Raman deviation from the typical nanocrystalline diamond films. The ferromagnetism we proposed arises from a crystal field established by the dipole moments on nitrogen which splits the degenerate energy levels. From the Raman spectrum, the structure of small grain-sized samples is more disordered than that of large grainsized samples, and the spectrum of small grain-sized samples is similar to that of the typical nanocrystalline diamond films. Finally, we find that the ferromagnetism induced from the structure distortion is dynamically dominant at low temperature, which leads to the result that the ferromagnetism of small grain-sized samples is slightly larger than that of large grain-sized ones.
Acknowledgments We would like to thank the support by the Ministry of Science and Technology of Republic of China for the Grant no. NSC-1012112-M-390-001-MY3 (Shih-Jye Sun), the Project no. P108/12/ G108 (GACR), LD14011 (HINT COST Action MP1202). The ion implantation was performed at the CANAM infrastructure of the NPI ASCR Rez.
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